Commercial vehicles, such as for example tractor trailers, often utilize air brake systems. A typical air brake system is capable of performing a service brake function and a park brake function. Service braking or normal braking refers to actuating the vehicle's brakes by depressing a brake pedal to deliver compressed air to a brake actuator. Service braking is used to slow the vehicle or bring the vehicle to a stop. Once stopped, the driver may wish to apply the park brakes to prevent the vehicle from rolling. Typical vehicle air brake systems utilize spring brakes to perform the park brake function. Spring brakes, as are known in the art, utilize spring force to engage the brakes and hold the vehicle stationary. Air pressure is used to disengage the brakes by compressing the spring in order to allow the vehicle to move.
Some vehicle air brake systems also include a work brake function for commercial vehicles that stop and start frequently, such as for example waste collection vehicles. Work braking refers to using a service brake application to perform a temporary park brake function. In other words, utilizing compressed air to apply the service brakes to keep the vehicle from rolling while the vehicle is parked. Utilizing service brakes in applications with frequent stopping and starting is preferable to utilizing park brakes because the service brakes are more durable and use less compressed air.
Typical work brake systems, however, are either not fail safe or have complicated fail safe systems. A fail safe work brake system will apply the park brakes if a particular vehicle condition exists or a contemplated specific system failure occurs, such as loss of compressed air. An example of a complex fail safe work brake system can be found in U.S. Pat. No. 5,458,402.
Furthermore, electro-pneumatic brake systems for commercial vehicles have been developed, though presently are not widely used. Present work brake systems, including known systems with complex pneumatic fail safe systems, have no provisions for providing fail safe operation in case of an electrical failure or driver error (e.g. the driver turns the ignition off while the work brake is engaged).